1. Field of the Invention
The present invention relates to electric engineering, in particular, to circuit breakers and, more specifically, to load cutouts.
2. Description of the Prior Art
The current state of electric engineering is characterized by rapid developments in power convertion engineering. Semiconductor converters, designed with thyristors and diodes, are highly sensitive to overloads and therefore require cutouts featuring very high tripping speed and currentlimiting capability for protection against overloads.
Known in the art is an automatic load cutout (SU, A, 296172) comprising a pair of contacts, one of which is equipped with a contact pressure means and rigidly fitted to an electrodynamic current limiter and electrically connected to it and the second of which is kinematically linked to its displacement and clamping assembly with this assembly kinematically linked to an emergency state sensor, an arc-extinguishing chamber, and two power terminals to accomodate the load. One power terminal is connected to the other contact of the pair of contacts, the other power terminal is electrically connected to the electrodynamic current limiter.
However, the high inertia of the contact displacement and clamping assembly in the known automatic load cutout does not allow the load to be cut out at the moment an emergency state (loverload or short circuit) sets in, this imparing the cutout's current-limiting capability and increasing the tripping time.
Besides this, to open the contacts of the pair of contacts by means of the electrodynamic current limiter the load current has to attain the operating current value at which the contacts open, this also impairing the current-limiting capability and increasing the operate time.
Known in the art is a current-limiting load cutout (FR, B, 8107360) comprising at least one pair of contacts, one contact of which is equipped with a contact pressure means and rigidly linked to an electrodynamic current limiter electrically connected to a controllable pulse current supply, and at least one emergency state sensor kinematically linked to the other contact of the contact pair, a device for extinguishing the arc occuring between the contacts of the contact pair, and a pair of power terminals per each contact pair, with one terminal of the pair electrically connected to the other contact of the contact pair.
The emergency state sensor is connected to one power terminal and to the controllable pulse current supply, the other power terminal being connected to the electrodynamic current limiter and to the controllable pulse current supply.
In the known current-limiting load cutout breaking the contacts of the contact pair, along with the mechanical forces generated by the contact displacement and clamping assembly, is provided by passing a current generated by the controllable pulse current supply through the contact pair and the electrodynamic current limiter.
In the known current-limiting load cutout the electrodynamic current limiter is connected into the load circuit and therefore carries the load current. At a given tripping speed, this results in impaired cost/efficiency due to greater dimensions, metal consumption, energy losses and high cost. Using the emergency state sensor in the switchable load circuit to open the contacts of the contact pair results in a longer tripping time of the contact displacement and clamping assembly due to a low sensitivity of such an emergency state sensor, this further impairing the cost/efficiency.